An electrically operated hydraulic pump is used for supplying operating oil as fluid to a clutch of an automatic transmission mounted on a vehicle such as an automobile, or for supplying cooling oil as fluid to an electric motor mounted on a hybrid type vehicle, and so on. A mechanical hydraulic pump, an electrically operated hydraulic pump, and so on can be employed as a pump mounted on a vehicle. The mechanical hydraulic pump uses driving force of an engine of the vehicle, while the driving force of the engine is not required to the electrically operated hydraulic pump. Before starting the engine, or immediately after the engine started, effective hydraulic pressure is not supplied from the mechanical hydraulic pump. Accordingly, there is a requirement to employ the electrically operated hydraulic pump in such situations.
A conventional electronic operated hydraulic pump is described in Japanese Patent No. 2002-317772 A (hereinafter, referred to as a reference 1). The electrically operated hydraulic pump according to the reference 1 includes a motor portion and a pump portion. The motor portion structures a sensor-less brushless DC motor. The pump portion absorbs and exhausts fluid by means of driving force of a rotating shaft driven by the motor portion. The motor portion includes a stator, a rotor and rotation controlling means. The stator includes plural exciting coils for generating a magnetic field. The rotor includes a magnet which faces the exciting coils and is arranged to be rotatable with the rotating shaft in a space inside of a resin-mold and closed-bottom cylindrical motor housing. The rotation controlling means controls rotation of the rotor by switching electric current of the exciting coils in accordance with a rotating position of the rotor. Further according to the electrically operated hydraulic pump in the reference 1, a fluid returning path is formed between the motor portion and the pump portion. Fluid flowing into a space inside the motor from the pump portion can be returned to the pump portion through the fluid returning path. In other words, the fluid circulates between the pump portion and the space inside the motor portion and therefore, the space of the motor portion, i.e., the stator and the rotor, is cooled. However, when temperature of the fluid is low and viscosity of the fluid is high, rotational resistance for the rotor may be increased because of the fluid adhering to the rotor inside the space of the motor.
As described above, according to the electrically operated hydraulic pump in reference 1, the sensor-less DC motor is employed so that there is an advantage that any particular apparatus for detecting a rotational position or the rotor position, or the like, may not be provided. Further, there is another advantage that the pump can be downsized because any particular apparatus for cooling the rotor of the motor portion, or the like, may not be provided.
The electrically operated hydraulic pump according to the reference 1 detects the rotational position of the rotor on the basis of speed electromotive force induced by the exciting coils. Accordingly, when the rotational speed of the rotor is high, detection precision of the rotational position of the rotor is also increased. However, when the rotation speed of the rotor is not high, the detection precision of the rotational position of the rotor may be decreased. Here, when the temperature of the fluid is low, the rotational speed of the rotor lowers because the viscosity of the fluid is high, i.e., because the fluid may become large rotational resistance to the rotor. Accordingly, when the temperature of the fluid is high, the detection precision of the rotational position of the rotor is increased, while when the temperature of the fluid is low, the detection precision of the rotor rotational position may be lowered. Specifically, before starting the engine or immediately after the engine started, i.e., before the motor is operated or immediately after the motor is operated, the temperature of the fluid may be low and the rotational speed of the rotor may not be increased because the viscosity of the fluid may be high. Accordingly, there is a possibility that the detection precision of the rotor is decreased. Consequently, the rotation of the motor may not be controlled adequately.
A need thus exists for an electrically operated hydraulic pump which is not susceptible to the drawback mentioned above.